The present invention relates to an improved process for the preparation of 4-nitro-o-xylene. More particularly the present invention relates to a process for the selective nitration of o-xylene to 4-nitro-o-xylene, wherein nitration of o-xylene is carried out in vapour phase over zeolite H-beta catalyst using dilute nitric acid as a nitrating agent. The process of present invention gives 4-nitro-o-xylene in with higher yields and selectivity in a continuous process without use of sulfuric acid making it an environment friendly process.
Nitration has been an active area of industrial chemistry for over a century. Nitration process is used for production of many large volume nitro aromatics, which are vital intermediates for dyes, pharmaceuticals, perfumes and pesticides.
Nitroxylenes are important because they are readily reduced to corresponding aminoxylenes (xylidines). 4-nitro o-xylene and 3-nitro o-xylenes are industrially important chemical intermediates for the manufacture of corresponding aminoxylenes (Xylidines). 4-amino o-xylene and 3-amino o-xylenes are starting materials for the production of riboflavin and agrochemicals respectively. As compared to 3-nitro-o-xylene, 4-nitro-o-xylene has better commercial and hence is desired by industry.
In the past the catalysts used for the nitration of o-xylene range from zeolites to metal oxides. Reference is made to U.S. Pat. No. 6,376,726 wherein a process for the nitration of o0xylene and other aromatic hydrocarbons is described in liquid phase using fuming nitric acid at reflux temperature and removal of water by Dean-Stark apparatus over modified clay catalysts but the selectivity for 4-nitro-o-xylene was low.
Another process for the nitration of o-xylene in liquid phase (Tomasz et. al. Synth. Commun. 31 (2), (2001) 173-1870) involves the use of 100% nitric acid as nitrating agent over silica supported solid acid catalysts at 20-60xc2x0 C. but the selectivity for 4-nitro o-xylene was lower. Another process for the nitration of o-xylene (Landau et. al. Catalysts today. 36 (1997) 497-510) is reported by using nitrogen dioxide as nitrating agent at 130xc2x0 C., WHSV=0.1 hxe2x88x921 over series of zeolites and sulfuric acid supported catalysts and in this case also the selectivity for 4 nitro o-xylene was low.
In the classical method of nitration of o-xylene is performed with nitric acid and environmentally hazardous sulfuric acid gives 3 nitro o-xylene (55%) and 4-nitro o-xylene (45%). One of the major disadvantages of this method is the formation of byproducts of polynitration and also causing environmental pollution during disposal of spent acid.
Most of the reported process have the drawbacks such as, catalyst not being selective for 4 nitro o-xylene, use of fuming nitric acid as a nitrating agent which leads to the oxidation products deactivating the catalyst. Nitrogen dioxide or mixture of sulphuric acid and nitric acid as nitrating agent which makes the process environmentally harmful, removal of water from the reaction mixture making the process tedious, formation of dinitroxylenes which causes problems during separation of the products.
Therefore, it is highly desired to develop a process for the selective preparation of 4-nitro-o-xylene by nitration of o-xylene using nitric acid as well as using solid catalyst to avoid the environmental problem associated with the use of sulphuric acid in the conventional process.
The main object of the present invention is to provide an improved process for the preparation of 4-nitro-o-xylene by vapor phase nitration of o-xylene using dilute nitric acid over large pore zeolites such as Y-zeolite, Mordenite and Beta zeolite which obviates the drawbacks as mentioned above.
Another object is the use of dilute nitric acid for the reaction, which is easy to handle.
Accordingly the present invention provides an improved process for the preparation of 4-nitro-o-xylene by reacting a preheated mixture of o-xylene and HNO3 over a solid catalyst followed by condensation to obtain the product, extracting the product with an organic solvent and neutralizing the product using a mild base.
In one embodiment of the present invention, the solid catalyst comprises a large pore zeolites selected from the group consisting of Y-Zeolites, Mordenite and H-beta.
In another embodiment of the invention, the catalyst comprises H-beta zeolite.
In another embodiment of the invention, nitric acid is used in an amount in the range of 10%-50%.
In a further embodiment of the invention, the amount of nitric acid used is about 30%.
In still another embodiment the molar ratio of nitric acid to o-xylene is in the range 2:1 to 1:2 preferably 1:1.5.
In another embodiment the nitration is carried out in an inert gas atmosphere said inert gas being selected from the group consisting of nitrogen, helium and argon.
In yet another embodiment of the invention, the reaction is conducted in a down flow reactor containing inert ceramic packing as preheater and N2 as a carrier gas for a period of 1 to 30 hours at 120 to 180xc2x0 C.
In yet another embodiment of the invention, condensation is carried out at a temperature in the range of 7-12xc2x0 C.
In yet another embodiment the reaction is carried out in vapor phase at a temperature in the range of 100 to 250xc2x0 C., preferably 150xc2x0 C.
In another embodiment the weight hourly space velocity expressed as grams of organic substrate per gram of catalyst per hour of the reaction mixture varies from 0.1 to 0.5.
In yet another embodiment of the invention, the selectivity for 4-nitro-o-xylene is in the range of 45 to 70%.